Hey there,
we’re in a similiar situation see How do you handle the additional costs of the new 8.6 Licence?
Until Camunda 8.5 you can use the basic Zeebee engine as Standalone in case you dont need Tools like operate, tasklist etc.
But with version 8.6. Also die Zeebe engine falls under the Enterprise licence and it seems like there is no free option of Camunda for commercial use.